1. Field of the Invention
The present invention relates to a process circuit for video signals to a monitor using a liquid crystal display (hereinafter referred to as LCD) panel as a display, and more specifically to a process circuit for improving contrast of the LCD panel.
2. Description of the Prior Art
Visual devices generally do not have a sufficient dynamic range for reproducing a wide range of contrast in natural light. Especially, in projection monitors using an LCD panel as a light valve, a sufficient dynamic range for tone reproduction is not attained due to on/off characteristics of the panel and restriction of the peak brightness of an external light source. An example of proposed methods for improving the contrast of such monitors by nonlinear process including electrical tone correction corresponding to a screen image is disclosed in Japanese Patent Publication No. 2-6069 (1990).
Active-matrix LCD panels employing a thin film transistor (TFT) as a switching element may reproduce half tone and have high picture quality and are thus widely used as a compact liquid crystal TV display or a light valve of a projection type TV. The TFT active-matrix LCD panel is driven in normally white mode to have a maximum transmittance when a difference between the level of the electrode signal and video signal such as luminance signal becomes minimum. Accordingly, the contrast is varied with the transmittance.
The transmittance of the LCD panel is, however, not linear against inputted video signals and the characteristics are different from .gamma. characteristics of a CRT (cathode ray tube). FIG. 1 is a graph showing an example of transmittance characteristics in normally-white mode, with transmittance as ordinate and difference (V) between the video signal voltage and the counter electrode voltage as abscissa.
Here the maximum transmittance of the LCD panel is determined to be 100% and the minimum to be 0%. As clearly seen in FIG. 1, the transmittance abruptly decreases against the difference in a range from 3V to 5V but gradually decreases in other range. Such correction as stretching of the black level of video signals is generally performed in order to obtain the linearity of the transmittance against the video signals. FIG. 2 is a graph showing an example of conventional black-level stretching, with output of video signal as ordinate and input of video signal as abscissa. The slope of the black level is made steeper than the other part to give linearity to the black level; that is, contrast in the black level is obtained.
The dynamic range of the LCD panel is, however, still far narrower than that of the CRT and can not reproduce the tone sufficiently. The maximum/minimum ratio of the transmittance of the LCD panel is smaller than that of the actual contrast and furthermore the tone is restricted by the light amount of an external light source.
Generally, tone should be corrected for each component signal, R(red), G(green) and B(blue), which requires a large scale of circuits and complicated adjustment. On the other hand, tone correction for only a luminance signal (Y) requires a smaller circuit scale and simpler adjustment while the effect is same as the tone corrections for R,G,B component signals. But since the larger the amounts of correction for the luminance signal are, the lesser the correlation of the luminance signal with a color signal (C) becomes, such an unbalanced picture is obtained that the contrast is sufficient but the color is too light.